Up to now, heating packets have been made of a heat generating composition, composed mainly of oxidative metal powders which generate heat upon contact with the oxygen in the air, which are enclosed in a gas permeable inner pouch, then sealed within an airtight outer pouch. These heating packets are used as medical devices, or as disposable pocket heaters for body warming. Furthermore, heating packets used for maintaining body temperature include the following: body heating packets used on the shoulders and lower back, pocket heating packets used in pockets and gloves, and shoe heating packets inserted in shoes.
The quantity of the heat generating composition, the proportions of the constituents of the heat generating composition, and the quantity of ventilation of the inner pouch of these heating packets are established according to usage, so that the heating packet attains the desired maximum temperature, time required to reach the desired temperature (rise time), and duration of heat production.
This heat generating composition is a mixture of oxidative metal powder, activated carbon, inorganic electrolytes, water, and so forth. Generally, iron powder is used as the oxidative metal powder. The heat generating composition generates heat when the oxidative metal powder becomes a metal oxide upon contact with the oxygen in the air.
Also, the gas permeable packing material used as the inner pouch may be one of the following: (1) a gas impermeable sheet wherein comparatively large holes, with equivalent diameters of 0.03-0.5 mm, are formed using needles or electric discharge; (2) nonwoven fabric, with limited gas permeability, formed by the superposition and thermocompression bonding of synthetic fibers of polyethylene, polypropylene, or the like; or (3) material (fine pored film) formed by dispersing a fine powder of calcium carbonate, barium sulfate, or the like in synthetic resin such as molten polyethylene, polypropylene, or the like, forming this into a film, then drawing the film to form fine pores with equivalent diameters of 10 .mu.m or less. Furthermore, nonwoven fabric or the like may be laminated on these gas permeable packing materials.
Here, the heating characteristics of the heating packet can be controlled by methods such as the following: the method of adjusting the proportions of the constituents of the heat generating composition as noted above; the method of controlling the quantity of oxygen supplied to the heat generating composition. Furthermore, the heating characteristics can be controlled by both the adjustment of the heat generating composition and the control of the quantity of ventilation through the gas permeable packing material. However, the heating characteristics are generally set by adjusting the quantity of ventilation through the gas permeable packing material used as the Inner pouch. Known methods are employed for measuring the quantity of ventilation through these gas permeable packing materials. Proposed methods include the following: (1) methods using the Gurley permeability measuring method (JIS P 8117) (Japanese Patent Publication No. 7-90030, Japanese Patent Laid-open Publication No. 8-80317), (2) methods using the Frazier-type tester (JIS L 1018, JIS L 1096) (Japanese Patent Laid-open Publication No. 7-67907), and (3) methods using the water vapor permeability measuring method (JIS Z 0208) (Japanese Patent Laid-open Publication No. 7-124192, Japanese Patent Laid-open Publication No.8-92075). These methods are used in the design and quality control of the heating characteristics of heating packets.
Also, the official methods for measuring the heating characteristics of heating packets are the testing methods prescribed in the Japanese Industrial Standards (JIS S 4100). According to these, the heating characteristics are measured using a temperature measuring apparatus prescribed in JIS S 4100, under conditions of 20.degree. C. external temperature and 65% relative humidity. The regulations prescribe that the heating characteristics be expressed as the time from the start of the heating operation until a temperature of 40.degree. C. is reached (rise time), the maximum temperature reached (maximum temperature), and the time for which temperatures of 40.degree. C. or more are maintained (continuation time).
The heating packet generates heat because of the oxidative reaction which occurs when oxygen is supplied by the oxygen in the air passing through the fine holes in the gas permeable packing material over a long period of time and the oxygen and oxidative metal powder (iron powder is a representative metal powder) come into contact.
However, the aforementioned method for measuring the quantity of ventilation depends on the size of the pores, form of the pores, type of material, and composition of the gas permeable packing material forming the inner pouch. There is no correlation between the measured value for the quantity of ventilation and the temperature characteristics. As a result, a problem is that the design and quality control for the heating packet are not adequate.
The aforementioned (1) Gurley gas permeability measurement method is constituted so that the weight of the gas chamber of the measurement apparatus operates as a pressure difference between the front and back of the gas permeable packing material. This is specifically a method for measuring the time necessary for a constant volume of gas to pass through the gas permeable packing material.
This measurement method is able to make the measurements for a packing material having relatively large pores, with equivalent diameters of 0.03-0.5 mm and formed with pins or electrical discharge, in several seconds to several minutes. However, a long period of time, such as several thousand to ten thousand seconds or more, is necessary for measuring a gas permeable packing material having a large number of small holes, such as a gas permeable packing material formed by dispersing a fine powder of calcium carbonate, barium sulfate, or the like in synthetic resin such as molten polyethylene, polypropylene, or the like, forming this into a film, then drawing the film to form fine pores with equivalent diameters of 10 .mu.m or less. Moreover, reproducibility is poor because the measured values fall outside the range (2-1800 seconds/100 ml) of the Gurley gas permeability measurement method itself.
Furthermore, there are cases where the same heating characteristics are given even when the Gurley gas permeability measurement values differ markedly depending on the type of gas permeable packing material. The problem is that there is no correlation between the values measured for the quantity of ventilation and heating characteristics. Also, the (2) method using a Frazier-type tester is a method for measuring mainly the gas permeability of textiles. This is a method which applies a pressure difference of 12.7 mmH.sub.2 O between the front and back surfaces of the gas permeable packing material. This method can be applied to the measurement of a gas permeable packing material with large air holes. However, the quantity of ventilation is too small in the case of a gas permeable packing material with fine pores of 5 .mu.m or less and this measurement method cannot measure outside of Its range (0.3-400 cc/cm.sup.2 /sec). Furthermore, (3) the water vapor permeability measurement method is a method for finding water vapor permeability from the amount of water vapor which diffuses through the gas permeable packing material under conditions of moisture saturation, without applying a pressure difference between the front and back surfaces of the gas permeable packing material. This appears to be a superior method. However, depending on the type of the gas permeable packing material, there is no correlation between the heating characteristics and the measured value of water vapor permeability. This applies in particular to the case of a gas permeable packing material which absorbs moisture or a gas permeable packing material having fine pores. Because of these issues, there is a strong desire for the development of a method for measuring the quantity of ventilation, which can measure the quantity of ventilation quickly and precisely, regardless of the type of gas permeable packing material, and with which a correlation with heating characteristics is attained.
Also, because of the lack of correlation between heating characteristics and quantity of ventilation of the gas permeable packing material used as the inner pouch as noted above, it has heretofore been impossible to design the heating characteristics with good precision and to have quality control for body heating packets, pocket heating packets, or shoe heating packets.
For this reason, it is desirable to develop heating packets having the desired heating characteristics and having stable heating characteristics.
In the case of shoe heating packets, the only heating packets available differ markedly from the desired heating characteristics, despite the extreme market requirements, for the aforementioned reasons. For this reason, it is desirable to develop heating packets having the desired heating characteristics and having stable heating characteristics. Therefore, it is an object of the present invention to provide a method for measuring the quantity of ventilation which correlates to heating characteristics and which can measure the quantity of ventilation quickly and precisely, regardless of the type of gas permeable packing material. It is another object of the present invention to provide an apparatus for measuring the quantity of ventilation which correlates to heating characteristics and which can measure the quantity of ventilation quickly and precisely, regardless of the type of gas permeable packing material.
Furthermore, it is another object of the present invention to provide a heating packet having the desired heating characteristics and having stable heating characteristics. It is another object of the present invention to provide various types of heating packets, as body warmers, pocket warmers, and shoe warmers.